A Cr-containing austenitic alloy has been used as various members because of being excellent in mechanical properties. In particular, since the members for a nuclear reactor is exposed to high-temperature water, a Cr-containing austenitic alloy excellent in corrosion resistance has been used as a member for a nuclear reactor. For example, as a member of a steam generator for a pressurized water reactor (PWR), a 60% Ni-30% Cr-10% Fe alloy or the like has been used.
These members are used in an environment of high-temperature water of about 300° C., which is a nuclear reactor water environment, for several years to several tens of years. For the Cr-containing austenitic alloy used as a steam generator tubing for nuclear plant, although Ni is contained much, and therefore the corrosion resistance is excellent and the corrosion rate is low, a minute amount of Ni is eluted from a base metal by the long-term use.
In a process in which reactor water circulates, the released Ni is carried to a reactor core portion and receives neutron irradiation in the vicinity of fuel. When receiving neutron irradiation, Ni is converted into radiocobalt by nuclear reaction. This radiocobalt continues to emit radioactive rays for a long period of time because the half-life thereof is very long. Therefore, if the amount of released Ni is large, regular inspection cannot be started until the radiation dose emitted decreases to a proper value, so that the period of regular inspection extends, which results in an economic loss.
To reduce the amount of released Ni is a very important issue in using a light water reactor for a long period of time. So far, therefore, by improving the corrosion resistance on the material side and by controlling the quality of reactor water, measures have been taken to suppress the elution of Ni from the Cr-containing austenitic alloy.
Patent Document 1 discloses a method in which Ni-based alloy heat-transfer pipe is annealed in the temperature range of 400 to 750° C. in an atmosphere having a degree of vacuum of 10−2 to 10−4 Torr to form an oxide film consisting mainly of chromium oxides, whereby the general corrosion resistance is improved.
Patent Document 2 discloses a method for producing a member for nuclear plant, in which after the solution treatment of a Ni-based precipitation strengthened alloy, heating treatment is performed as at least part of age hardening treatment and oxide film forming treatment in an oxidizing atmosphere of 10−3 Torr to atmospheric pressure.
Patent Document 3 discloses a method for producing a Ni-based alloy product, in which a Ni-based alloy product is heat-treated in an atmosphere of hydrogen or a mixed atmosphere of hydrogen and argon, the atmosphere having a dew point of −60° C. to +20° C.
Patent Document 4 discloses a method for forming a chromium-rich layer by exposing an alloy workpiece containing nickel and chromium to a gas mixture of water vapor and at least one kind of non-oxidizing gases.
Patent Documents 5 and 6 disclose a method for producing a Cr-containing nickel-based alloy pipe, in which the Cr-containing nickel-based alloy pipe is treated in an atmosphere consisting of non-oxidizing gas containing oxidizing gas, whereby a chromium oxide film having a predetermined thickness is formed on the inner surface of pipe.